Welding helmets fitted with a filter for protecting welders from optical radiation generated during welding operations are well known. A typical welding helmet filter is a device disposed in an aperture in the front of a welding helmet that a welder can look through to view welding operations, without their eyes being exposed to harmful amounts of optical radiation.
The visible band of electromagnetic spectrum, that is the range of optical radiation that can be detected by the human eye, is generally understood in the welding industry to be the band of wavelengths from approximately 380 to 780 nanometres (nm). A further band of wavelengths of optical radiation having wavelengths shorter than the visible band is often referred to as ultraviolet (UV) radiation. The part of UV radiation spectrum that is of interest in the welding industry is approximately 200 nm to 380 nm. A further band of wavelengths of optical radiation having wavelengths longer than the visible band is often known as infrared radiation (IR). The part of IR radiation spectrum that is of interest in the welding industry is approximately 780 nm to 3000 nm.
Ultraviolet and infrared radiation often generated by a welding arc during welding processes can be harmful to the human eye, potentially causing irreversible damage. Further, the visible radiation generated by welding processes is often too bright to be comfortable for the welder to observe directly and hence a means of attenuating the transmission of this visible radiation is desirable. Generally, the human eye is protected from bright visible radiation by normal aversion response, such as blink reflex or head movement, however, in some circumstances, bright visible radiation can dazzle the welder and cause temporary blindness.
Known welding helmet filter types include passive filters or automatically darkening filters (ADFs). A passive filter is often a dark coloured, usually green, glass lens which allows a limited range of wavelengths of radiation to pass through with a certain level of transmittance. A passive filter is often rather dark and provides its user with only a limited amount of visibility in normal ambient light conditions and as such is usually positioned in front of the welders face immediately before the welder starts the welding operation. A traditional ADF has a default state that is a light state, such that the welder has good forward visibility. It automatically changes to a darker state at the start of welding operations and returns to the light state when the welder ceases the welding operations. Often, ADFs use switchable electro-optical technologies such as liquid crystal technology to provide these different states. Examples of liquid crystal technology for use in an ADF are described in U.S. Pat. Nos. 6,097,451, 5,825,441, and 7,477,330.
Generally available welding helmet filters have a certain shade or range of shades or shade numbers. Shade S is related to luminous transmittance, TL (expressed as a fraction of light passing through a filter with respect to a standard light source and a standard observer) by the following equation:S=1+(7/3)10 log(1/TL)
Here, TL is defined as the ratio of transmitted luminous intensity to the incident luminous intensity. A precise definition of luminous transmittance is given in various standards, for example the European standard for Personal Eye Protection—Transmittance requirements, EN 169:2002. For given shade numbers, the tolerance in luminous transmittance and maximum transmittance for different wavelength bands of UV and IR radiation are usually specified by industrial standards that are applied to welding filters. For example, shade three (3), in the European standard for Personal Eye Protection—Transmittance requirements, EN 169:2002, specifies a luminous transmittance level of between 8.5 and 17.8%. By way of further example, shade twelve (12) specifies a luminous transmittance level of between 0.0012 and 0.0032%. For shade 12, a maximum permitted UV transmittance at wavelengths between 313 nm and 365 nm is 0.0012% and a maximum allowable mean spectral IR transmittance for the band of wavelengths from 780 to 1400 nm is 0.027%. The maximum UV and IR transmittance values specified for the darkest possible shade for a particular filter (e.g., shade 12) must also hold also for the lightest shade (e.g., shade 3).
The shade of a typical passive filter for arc welding is shade ten (10). The shade of a typical ADF during the light state shade 3, whereas the shade in the dark state can often be pre-selected by the welder and is typically in the range between shade eight (8) and shade twelve (12). The dark state shade is chosen depending on the type of welding operation being conducted and personal preference or comfort of the welder.
Providing welding helmets fitted with an ADF allows the welder to have some vision whilst they are not welding without having to raise the face shield or remove the welding helmet, however, it does not take account of the welder's peripheral vision. More recently, some welding helmets have been fitted with passive side filters, often known as side windows to improve the welder's peripheral vision. These side windows are passive filters that reduce the level of visible radiation that is transmitted to the welder. Typically, the transmittance of currently available passive filters is comparable to a level between the dark and light states of automatically darkening welding filters. The shade or level of transmittance chosen is a balance between allowing sufficient visible radiation transmittance to improve the welder's peripheral vision and minimizing the amount of visible radiation entering the helmet that could cause glare or internal reflections on the main filter during welding operations. Welding helmets with such side windows are described in U.S. Pat. No. 5,191,468.
Other side window configurations known in the art, for example, those described in US 2004/0117888, include transparent side windows covered with doors that can be opened when the welder is not performing welding operations and closed manually by the welder prior to welding operations. The side windows are typically located on either side of the main filter, that is the filter in the front of the welding helmet that the welder uses for viewing welding operations, and therefore can improve the welder's peripheral vision when the welder is not welding.
Side windows improve welder's peripheral vision and awareness considerably. However there are certain situations where the functionality of a welding helmet with passive side windows may be further improved to better meet the needs of a welder. For example, further optimizing a welder's peripheral vision can greatly improve the welder's experience of use of the helmet and the welding process.